Link Layer

2

Content

Error detection and correction MAC sub-layer Ethernet Token Ring

3

Access Protocols

Who gets to use the channel next? Fixed/Static assignment Demand assignment Contention Turn-Based

4

Contention Access Protocols

No coordination between hosts Control is completely distributed Outcome is probabilistic Examples: ALOHA, CSMA, CSMA/CD

5

Contention Access (cont’d)

Advantages: Short delay for bursty traffic Simple (due to distributed control) Flexible to fluctuations in the number of hosts Fairness

6

Contention Access (cont’d)

Disadvantages: Can not be certain who will acquire the

media/channel Low channel efficiency with a large number of

hosts Not good for continuous traffic (e.g., voice) Cannot support priority traffic High variance in transmission delays

7

Contention Access Methods

Pure ALOHA Slotted ALOHA CSMA

1-Persistent CSMA Non-Persistent CSMA P-Persistent CSMA

CSMA/CD

8

Slotted ALOHAAssumptions all frames same size time is divided into equal

size slots, time to transmit 1 frame

nodes start to transmit frames only at beginning of slots

nodes are synchronized if 2 or more nodes

transmit in slot, all nodes detect collision

Operation when node obtains fresh

frame, it transmits in next slot

no collision, node can send new frame in next slot

if collision, node retransmits frame in each subsequent slot with prob. p until success

9

Slotted ALOHA

Pros single active node can

continuously transmit at full rate of channel

highly decentralized: only slots in nodes need to be in sync

simple

Cons collisions, wasting slots idle slots nodes may be able to

detect collision in less than time to transmit packet

clock synchronization

10

Slotted Aloha efficiency

Suppose N nodes with many frames to send, each transmits in slot with probability p

prob that node 1 has success in a slot = p(1-p)N-1

prob that any node has a success = Np(1-p)N-1

For max efficiency with N nodes, find p* that maximizes Np(1-p)N-1

For many nodes, take limit of Np*(1-p*)N-1 as N goes to infinity, gives 1/e = .37

Efficiency is the long-run fraction of successful slots when there are many nodes, each with many frames to send

At best: channelused for useful transmissions 37%of time!

11

Pure (unslotted) ALOHA unslotted Aloha: simpler, no synchronization when frame first arrives

transmit immediately

collision probability increases: frame sent at t0 collides with other frames sent in [t0-1,t0+1]

12

Pure Aloha efficiency

P(success by given node) = P(node transmits) .

P(no other node transmits in [p0-1,p0] .

P(no other node transmits in [p0-1,p0]

= p . (1-p)N-1 . (1-p)N-1

= p . (1-p)2(N-1)

… choosing optimum p and then letting n -> infty ...

= 1/(2e) = .18 Even worse !

13

Carrier Sense Multiple Access (CSMA)

We could achieve better throughput if we could listen to the channel before transmitting a packet

This way, we would stop avoidable collisions. To do this, we need “Carrier Sense Multiple

Access,” or CSMA, protocols

14

Assumptions with CSMA Networks

1. Constant length packets

2. No errors, except those caused by collisions

3. No capture effect

4. Each host can sense the transmissions of all other hosts

5. The propagation delay is small compared to the transmission time

15

CSMA collisionscollisions can still occur:propagation delay means two nodes may not heareach other’s transmissioncollision:entire packet transmission time wasted

spatial layout of nodes

note:role of distance & propagation delay in determining collision probability

16

CSMA (cont’d)

There are several types of CSMA protocols: 1-Persistent CSMA Non-Persistent CSMA P-Persistent CSMA

17

1-Persistent CSMA

Sense the channel. If busy, keep listening to the channel and transmit

immediately when the channel becomes idle. If idle, transmit a packet immediately.

If collision occurs, Wait a random amount of time and start over again.

18

1-Persistent CSMA (cont’d)

The protocol is called 1-persistent because the host transmits with a probability of 1 whenever it finds the channel idle.

19

The Effect of Propagation Delayon CSMA

A Bcarrier sense = idle

Transmit a packet

Collision

packet

20

Propagation Delay and CSMA

Contention (vulnerable) period in Pure ALOHA two packet transmission times

Contention period in Slotted ALOHA one packet transmission time

Contention period in CSMA up to 2 x end-to-end propagation delay

Performance of CSMA > Performance of Slotted ALOHA >

Performance of Pure ALOHA

21

1-Persistent CSMA (cont’d)

Even if prop. delay is zero, there will be collisions Example:

If stations B and C become ready in the middle of A’s transmission, B and C will wait until the end of A’s transmission and then both will begin transmitted simultaneously, resulting in a collision.

If B and C were not so greedy, there would be fewer collisions

22

Non-Persistent CSMA

Sense the channel. If busy, wait a random amount of time and sense the

channel again If idle, transmit a packet immediately

If collision occurs wait a random amount of time and start all over again

23

Tradeoff between 1- and Non-Persistent CSMA

If B and C become ready in the middle of A’s transmission, 1-Persistent: B and C collide Non-Persistent: B and C probably do not collide

If only B becomes ready in the middle of A’s transmission, 1-Persistent: B succeeds as soon as A ends Non-Persistent: B may have to wait

24

P-Persistent CSMA

Optimal strategy: use P-Persistent CSMA Assume channels are slotted One slot = contention period (i.e., one round

trip propagation delay)

25

P-Persistent CSMA (cont’d)

1. Sense the channel If channel is idle, transmit a packet with probability p

if a packet was transmitted, go to step 2 if a packet was not transmitted, wait one slot and go to step 1

If channel is busy, wait one slot and go to step 1.

2. Detect collisions If a collision occurs, wait a random amount of time and go to step

1

26

P-Persistent CSMA (cont’d)

Consider p-persistent CSMA with p=0.5 When a host senses an idle channel, it will only

send a packet with 50% probability If it does not send, it tries again in the next slot.

27

Comparison of CSMA and ALOHA Protocols

(Number of Channel Contenders)

28

CSMA/CD

In CSMA protocols If two stations begin transmitting at the same time, each

will transmit its complete packet, thus wasting the channel for an entire packet time

In CSMA/CD protocols The transmission is terminated immediately upon the

detection of a collision CD = Collision Detect

29

CSMA/CD (Collision Detection)

collision detection: easy in wired LANs: measure signal strengths,

compare transmitted, received signals difficult in wireless LANs: receiver shut off while

transmitting

human analogy: the polite conversationalist

30

CSMA/CD collision detection

31

CSMA/CD

Sense the channel If idle, transmit immediately If busy, wait until the channel becomes idle

Collision detection Abort a transmission immediately if a collision is detected Try again later after waiting a random amount of time

32

CSMA/CD (cont’d)

Carrier sense reduces the number of collisions

Collision detection reduces the effect of collisions, making the

channel ready to use sooner

33

Collision detection time

How long does it take to realize there has been a collision?

Worst case: 2 x end-to-end prop. delay

A B

packet

34

Turn-Based Access Protocols

A

D

C

B

35

IEEE 802 LANs

LAN: Local Area Network What is a local area network?

A LAN is a network that resides in a geographically restricted area

LANs usually span a building or a campus

36

Characteristics of LANs

Short propagation delays

Small number of users

Single shared medium (usually)

Inexpensive

37

Common LANs

Bus-based LANs Ethernet (*) Token Bus (*)

Ring-based LANs Token Ring (*)

Switch-based LANs Switched Ethernet ATM LANs

(*) IEEE 802 LANs

38

IEEE 802 Standards

802.1: Introduction

802.2: Logical Link Control (LLC)

802.3: CSMA/CD (Ethernet)

802.4: Token Bus

802.5: Token Ring

802.6: DQDB

802.11: CSMA/CA (Wireless LAN)

39

IEEE 802 Standards (cont’d)

802 standards define: Physical layer protocol

Data link layer protocol Medium Access (MAC) Sublayer

Logical Link Control (LLC) Sublayer

40

OSI Layers and IEEE 802

802.2 Logical Link Control

802.3 802.4 802.5Medium Access Control

Data Link Layer

Physical Layer

Higher Layers

OSI layers IEEE 802 LAN standards

Higher Layers

CSMA/CD Token-passing Token-passing bus bus ring

41

IEEE 802 LANs (cont’d)

Ethernet Token Ring

42

Ethernet (CSMA/CD)

IEEE 802.3 defines Ethernet Layers specified by 802.3:

Ethernet Physical Layer Ethernet Medium Access (MAC) Sublayer

43

Ethernet (cont’d)

Possible Topologies:

1. Bus

2. Branching non-rooted tree for large Ethernets

44

Minimal Bus Configuration

Host

Transceiver

TransceiverCable

Coaxial Cable

Terminator

45

Typical Large-Scale Configuration

Host

Repeater

Ethernetsegment

46

Ethernet Physical Layer

Transceiver Transceiver Cable

4 Twisted Pairs 15 Pin Connectors

Channel Logic Manchester Phase Encoding 64-bit preamble for synchronization

47

Ethernet Cabling Options

10Base5: Thick Coax 10Base2: Thin Coax (“cheapernet”) 10Base-T: Twisted Pair 10Base-F: Fiber optic Each cabling option carries with it a different set of

physical layer constraints (e.g., max. segment size, nodes/segment, etc.)

48

Ethernet Physical Configuration

For thick coaxial cable Segments of 500 meters maximum Maximum total cable length of 1500 meters

between any two transceivers Maximum of 2 repeaters in any path Maximum of 100 transceivers per segment Transceivers placed only at 2.5 meter marks on

cable

49

Manchester Encoding

1 bit = high/low voltage signal 0 bit = low/high voltage signal

1 0 1 1 0 0Data stream

Encodedbit pattern

50

Ethernet Synchronization

64-bit frame preamble used to synchronize reception

7 bytes of 10101010 followed by a byte containing 10101011

Manchester encoded, the preamble appears like a sine wave

51

Ethernet: MAC Layer

Data encapsulation Frame Format Addressing Error Detection

Link Management CSMA/CD Backoff Algorithm

52

Frame Check Seq.(4 bytes)

MAC Layer Ethernet Frame Format

Destination(6 bytes)

Length (2 bytes)

Data(46-1500 bytes)

Pad

Source(6 bytes)

Multicast bit

53

Ethernet MAC Frame Address Field

Destination and Source Addresses: 6 bytes each

Two types of destination addresses Physical address: Unique for each user Multicast address: Group of users First bit of address determines which type of address is

being used

0 = physical address

1 = multicast address

54

Ethernet MAC FrameOther Fields

Length Field 2 bytes in length determines length of data payload

Data Field: between 0 and 1500 bytes Pad: Filled when Length < 46 Frame Check Sequence Field

4 bytes Cyclic Redundancy Check (CRC-32)

55

CSMA/CD

Recall: CSMA/CD is a “carrier sense” protocol.

If channel is idle, transmit immediately If busy, wait until the channel becomes idle

CSMA/CD can detect collections. Abort transmission immediately if there is a collision Try again later according to a backoff algorithm

56

Ethernet Backoff Algorithm:Binary Exponential Backoff

If collision, Choose one slot randomly from 2k slots, where k is the

number of collisions the frame has suffered. One contention slot length = 2 x end-to-end propagation

delay

This algorithm can adapt to changes in network load.

57

Binary Exponential Backoff (cont’d)

slot length = 2 x end-to-end delay = 50 s

A B

t=0s: Assume A and B collide (kA = kB = 1)A, B choose randomly from 21 slots: [0,1]Assume A chooses 1, B chooses 1

t=100s: A and B collide (kA = kB = 2)A, B choose randomly from 22 slots: [0,3]Assume A chooses 2, B chooses 0

t=150s: B transmits successfullyt=250s: A transmits successfully

58

Binary Exponential Backoff (cont’d)

In Ethernet, Binary exponential backoff will allow a maximum

of 15 retransmission attempts If 16 backoffs occur, the transmission of the frame

is considered a failure.

59

Ethernet Performance

60

Ethernet Features and Advantages

1. Passive interface: No active element

2. Broadcast: All users can listen

3. Distributed control: Each user makes own decision

Simple

Reliable

Easy to reconfigure

61

Ethernet Disadvantages

Lack of priority levels

Cannot perform real-time communication

Security issues

62

Hubs, Switches, Routers

Hub: Behaves like Ethernet

Switch: Supports multiple collision domains A collision domain is a segment

Router: operates on level-3 packets

63

Why Ethernet Switching?

LANs may grow very large The switch has a very fast backplane It can forward frames very quickly to the

appropriate subnet Cheaper than upgrading all host interfaces to

use a faster network

64

Ethernet Switching

Connect many Ethernet through an “Ethernet switch”

Each Ethernet is a “segment” Make one large, logical segment

to segment 1

to segment 2to segment 3

to segment 4

65

Collision Domains

Host

switch

EthernetHub

AB

C

D

E

FA,B,C D,E,F

G

H

Z Each segment runs a standardCMSA protocol

66

Layer-2 routing tables

Host

switch

EthernetHub

AB

C

D

E

FA,B,C D,E,F

G

H

Z Switch must forward packets fromA,B,C to the other segment

Switch builds a large table

For each packet, look up in table and maybe forward the packet

67

Learning MAC addresses

Host

switch

Ethernetsegment

AB

C

D

E

FA,B,C D,E,F

Per-port routing tableG

H

Z

Switch adds hosts to routing table when it sees a packet with a given source address

68

Spanning Trees

Want to allow multiple switches to connect together

What If there is a cycle in the graph of switches connected together? Can’t have packets circulate forever! Must break the cycle by restricting routes

69

Spanning Trees

Host

switchesAB

C

D

E

F

G

H

Z J

k

1 2

3

70

Spanning Trees

Host

switchesAB

C

D

E

F

G

H

Z J

k

1 2

3no cyclesin the graph of switches

71

Spanning Tree Protocol

1. Each switch periodically sends a configuration message out of every port. A message contains: (ID of sender, ID of root, distance from sender to root).

2. Initially, every switch claims to be root and sends a distance field of 0.3. A switch keeps sending the same message (periodically) until it hears a “better”

message.4. “Better” means:

• A root with a smaller ID• A root with equal ID, but with shorter distance• The root ID and distance are the same as we already have, but the sending bridge

has a smaller ID.5. When a switch hears a better configuration message, it stops generating its own

messages, and just forwards ones that it receives (adding 1 to the distance).6. If the switch realizes that it is not the designated bridge for a segment, it stops sending

configuration messages to that segment.

Eventually:• Only the root switch generates configuration messages,• Other switches send configuration messages to segments for which they are the

designated switch

72

Token Ring

IEEE 802.5 Standard Layers specified by 802.5:

Token Ring Physical Layer Token Ring MAC Sublayer

73

Token Ring (cont’d)

Token Ring, unlike Ethernet, requires an active interface

HostRinginterface

74

Token Ring Physical Layer

Ring Interfaces Listen and Transmit Modes

Channel Logic Differential Manchester Encoding

75

Token Ring Interface Modes

Tostation

Fromstation

Tostation

Fromstation

ListenMode

TransmitModeone-bit

delay

76

Differential Manchester Encoding

1 0 0 1 1

Transitions take place at midpoint of interval 1 bit: the initial half of the bit interval carries the same

polarity as the second half of the previous interval 0 bit: a transition takes place at both the beginning and the

middle of the bit interval Differential Manchester is more efficient than standard

Manchester encoding

77

Token Ring MAC Sublayer

Token passing protocol Frame format Token format

78

Token Passing Protocol

A token (8 bit pattern) circulates around the ring Token state:

Busy: 11111111 Idle: 11111110

79

Token Passing Protocol (cont’d)

General Procedure: Sending host waits for and captures an idle token Sending host changes the token to a frame and circulates

it Receiving host accepts the frame and continues to

circulate it Sending host receives its frame, removes it from the ring,

and generates an idle token which it then circulates on the ring

80

Token Ring Frame and Token Formats

SD AC ED

SC AC FC DestinationAddress

SourceAddress Data Checksum ED FS

Token Format

Frame Format

1 1 1

1 1 1 2/6 2/6 unlimited 4 1 1

Bytes

81

Token Ring Delimiters

SD = Starting Delimiter ED = Ending Delimiter They contains invalid differential Manchester codes

SD AC ED

SC AC FC DestinationAddress

SourceAddress Data Checksum ED FS

82

Token Ring Access Control Field

P = Priority bits provides up to 8 levels of priority when accessing the ring

T = Token bit T=0: Token T=1: Frame

SD AC ED

P P P T M R R R

(Note: The AC field is also used in frames)

83

Token Ring Access Control Field (cont’d)

SD AC ED

P P P T M R R R

M = Monitor Bit Prevents tokens and frames from circulating indefinitely All frames and tokens are issued with M=0 On passing through the “monitor station,” M is set to 1 All other stations repeat this bit as set A token or frame that reaches the monitor station with M=1 is

considered invalid and is purged

84

The Token Ring Monitor Station

One station on the ring is designated as the “monitor station”

The monitor station: marks the M bit in frames and tokens removes marked frames and tokens from the ring watches for missing tokens and generates new ones after

a timeout period

85

Token Ring Access Control Fields (cont’d)

SD AC ED

P P P T M R R R

R = Reservation Bits Allows stations with high priority data to request (in frames

and tokens as they are repeated) that the next token be issued at the requested priority

86

Token Ring Frame Control Field

FC = Frame Control Field Defines the type of frame being sent Frames may be either data frames or some type of control

frame. Example control frames: Beacon: Used to locate breaks in the ring Duplicate address test: Used to test if two stations have the

same address

SC AC FC DestinationAddress

SourceAddress Data Checksum ED FS

87

Token Ring Address & Data Fields

Address Fields: Indicate the source and destination hosts Broadcast:

Set all destination address bits to 1s. Data

No fixed limit on length Caveat: Hosts may only hold the token for a limited amount

of time (10 msec)

SC AC FC DestinationAddress

SourceAddress Data Checksum ED FS

88

Token Ring Checksum and Frame Status

Checksum: 32-bit CRC FS = Frame Status

Contains two bits, A and C When the message arrives at the destination, it sets A=1 When the destination copies the data in the message, it

sets C=1

SC AC FC DestinationAddress

SourceAddress Data Checksum ED FS

89

Using Priority in Token Ring

If a host wants to send data of priority n, it may only grab a token with priority value n or lower.

A host may reserve a token of priority n by marking the reservation bits in the AC field of a passing token or frame Caveat: The host may not make the reservation if the token or

frame’s AC field already indicates a higher priority reservation The next token generated will have a priority equal to the highest

reserved priority

90

Priority Transmission: Example

AD

C

B

Host B has 1 frame of priority 3 to send to AHost C has 1 frame of priority 2 to send to AHost D has 1 frame of priority 4 to send to AToken starts at host A with priority 0 and circulates

clockwiseHost C is the monitor station

91

Example (cont’d)

Event Token/Frame AC FieldA generates a token P=0, M=0, T=0, R=0B grabs the token and sets themessage destination to A P=3, M=0, T=1, R=0Frame arrives at C, and C reservespriority level 2. Monitor bit set. P=3, M=1, T=1, R=2Frame arrives at D, andD attempts to reserve priority level 4: P=3, M=1, T=1, R=4Frame arrives at A, and Acopies it P=3, M=1, T=1, R=4Frame returns to B, so B removesit, and generates a new token P=4, M=0, T=0, R=0Token arrives at C, but its priority istoo high. C reserves priority 2. M bit. P=4, M=1, T=0, R=2

92

Example (cont’d)Event Token/Frame AC FieldToken arrives at D, and D grabsit, sending a message to A P=4, M=0, T=1, R=2Frame arrives at A, and A copies it P=4, M=0, T=1, R=2Frame arrives at B, which doesnothing to it P=4, M=0, T=1, R=2Frame arrives at C, which sets themonitor bit P=4, M=1, T=1, R=2Frame returns to D, so D removesit and generates a new token with P=2 P=2, M=0, T=0, R=0

etc… Attempt to complete this scenario on your own.